Challenges for the future of allogeneic hematopoietic cell transplantation include reducing the morbidity of the procedure and maximizing the anti-tumor activity of hematopoietic grafts. Hematopoietic stem cells (HSCs) and other defined populations can now be isolated from mice and human bone marrow and mobilized peripheral blood. Transplantation of purified HSCs will eliminate the complication of graft-vs-host disease (GVHD). However, such grafts may result in other problems including failure to engraft and loss of graft-vs-tumor (GVT) effects. Studies focus on transplantation of purified hematopoietic cell populations in preclinical mouse models. From studies previously performed under funding from this Program Project Grant we addressed these potential problems and were able to achieve reproducible and sustained engraftment of purified HSCs transplanted across even the most severe immune barriers. Furthermore, we established a model for testing GVT candidate populations in co-transfer studies with HSCs. In the current Project we propose to expand upon our prior studies. Specifically, we aim to further understand the cellular basis of resistance to engraftment and design regimens that can be utilized to confer sustained hematopoiesis with significant anti-tumor effects in recipients prepared with both myeloablative and non-myeloablative regimens. This Project contributes directly to understanding the basic biology of clinical studies proposed in Project 1 and Project 0013. Project will interact significantly with Project 0009. In the study of the in vivo location of the barrier to allogeneic HSC engraftment, and will interact with both Projects in the design of regimens that will confer GVT activity. The Project will be supported by Cores. 9002 will be especially important in aiding in the delineation of homing and expansion patterns of HSCs and in tracking the effects of GVT candidate cells co-transferred with HSCs.